A few bugfixes for mails (serious performance impact!)

Just a few bug fixes. Performance impact is very significant - here the speed of sending an e-mail went from >25s to <2s:

ActiveUp.Net.Mail.Validator.GetMxRecords(...) -- add 'if (answer == null) { return null; } else { ... }' . It now generates a null reference exception which is caught in another GetMxRecords method. That one should also handle a null check properly.

DNS resolves from DirectSend are incorrect. It does a ServerCollection.Add in the DirectSend method, which defaults to port 25 for DNS -- this must be port 53; otherwise you will get a timeout (making sending mails very, very, very slow)

Listing name servers should be unique. DnsQuery.GetMachineDnsServers returns a list of all DNS servers,
but if you have multiple NIC's, there is probably an overlap. Best to use a HashSet<string> and sort the list to ensure you try to connect to IPv4 addresses first.

DnsQuery.QueryServer uses InterNetwork for the Socket connection. This should be dnsServer.AddressFamily to also support f.ex. IPv6.